Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. About 50% of the refinery gasoline blending pool in the United States is produced by this process, with almost all being produced using the fluid catalytic cracking (FCC) process. In the FCC process, heavy hydrocarbon fractions are converted into lighter products by reactions taking place at high temperatures in the presence of a catalyst, with the majority of the conversion or cracking occurring in the gas phase. The FCC hydrocarbon feedstock (feedstock) is thereby converted into gasoline and other liquid cracking products as well as lighter gaseous cracking products of four or fewer carbon atoms per molecule. These products, liquid and gas, consist of saturated and unsaturated hydrocarbons.
In FCC processes, feedstock is injected into the riser section of a FCC reactor, where the feedstock is cracked into lighter, more valuable products upon contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator. As the endothermic cracking reactions take place, carbon is deposited onto the catalyst. This carbon, known as coke, reduces the activity of the catalyst and the catalyst must be regenerated to revive its activity. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the FCC reactor, where they are separated. Subsequently, the catalyst flows into a stripping section, where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection. Following removal of occluded hydrocarbons from the spent cracking catalyst, the stripped catalyst flows through a spent catalyst standpipe and into a catalyst regenerator.
Typically, catalyst is regenerated by introducing air into the regenerator and burning off the coke to restore catalyst activity. These coke combustion reactions are highly exothermic and as a result, heat the catalyst. The hot, reactivated catalyst flows through the regenerated catalyst standpipe back to the riser to complete the catalyst cycle. The coke combustion exhaust gas stream rises to the top of the regenerator and leaves the regenerator through the regenerator flue. The exhaust gas generally contains nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO), oxygen (O2), HCN or ammonia, nitrogen and carbon dioxide (CO2).
The three characteristic steps of the FCC process that the cracking catalyst undergoes can therefore be distinguished: 1) a cracking step in which feedstock is converted into lighter products, 2) a stripping step to remove hydrocarbons adsorbed on the catalyst, and 3) a regeneration step to burn off coke deposited on the catalyst. The regenerated catalyst is then reused in the cracking step.
A major breakthrough in FCC catalysts came in the early 1960's, with the introduction of molecular sieves or zeolites. These materials were incorporated into the matrix of amorphous and/or amorphous/kaolin materials constituting the FCC catalysts of that time. These new zeolitic catalysts, containing a crystalline aluminosilicate zeolite in an amorphous or amorphous/kaolin matrix of silica, alumina, silica-alumina, kaolin, clay or the like were at least 1,000-10,000 times more active for cracking hydrocarbons than the earlier amorphous or amorphous/kaolin containing silica-alumina catalysts. This introduction of zeolitic cracking catalysts revolutionized the fluid catalytic cracking process. New processes were developed to handle these high activities, such as riser cracking, shortened contact times, new regeneration processes, new improved zeolitic catalyst developments, and the like.
The new catalyst developments revolved around the development of various zeolites such as synthetic types X and Y and naturally occurring faujasites; increased thermal-steam (hydrothermal) stability of zeolites through the inclusion of rare earth ions or ammonium ions via ion-exchange techniques; and the development of more attrition resistant matrices for supporting the zeolites. The zeolitic catalyst developments gave the petroleum industry the capability of greatly increasing throughput of feedstock with increased conversion and selectivity while employing the same units without expansion and without requiring new unit construction.
After the introduction of zeolite containing catalysts the petroleum industry began to suffer from a lack of crude availability as to quantity and quality accompanied by increasing demand for gasoline with increasing octane values. The world crude supply picture changed dramatically in the late 1960's and early 1970's. From a surplus of light-sweet crudes the supply situation changed to a tighter supply with an ever-increasing amount of heavier crudes, such as petroleum residues, having a higher sulfur content.
Petroleum resid(ue) is the heavy fraction remaining after distillation of petroleum crudes at atmospheric pressure (atmospheric resid) or at reduced pressure (vacuum resid). Resids have a high molecular weight and most often contain polycyclic aromatic hydrocarbons (PAH's). These molecules have more than 3-4 aromatic rings and provide the greatest limitation to the conversion of the resids into the desired products. This is because of their high stability and the lack of sufficient hydrogen in the ring structures to be converted to smaller more useful molecules. Moreover, the desired products, e.g. transportation fuels, are limited to alkylated single aromatic rings. No matter which type of resid conversion process is applied, a substantial fraction of resid molecules have fragments, which can be cracked off to become liquids (or gas) in the transportation fuels and vacuum oil boiling range. The aromatic cores cannot be cracked under FCC cracking conditions (in order to also remove these species hydrocracking must be considered). Therefore, one should not attempt to overly convert resids because then the selectivity will shift to the thermodynamically favored, but lower valued products: coke and gaseous hydrocarbons. As a result, gasoline yields are lower in residue such as FCC processing. These heavier and high sulfur crudes and residues present processing problems to the petroleum refiner in that these heavier crudes invariably also contain much higher metals with accompanying significantly increased asphaltic content. Typical contaminant metals are nickel, vanadium, and iron.
It has long been known that topped crudes, residual oils and reduced crudes with high levels of contaminating metals reduce the refiners selectivity to valuable transportation fuels and FCC catalysts can be deactivated at relatively high metal concentrations, e.g., 5,000-10,000 ppm in combination with elevated regenerator temperatures. In particular, when reduced crude containing feeds with high vanadium and nickel levels are processed over a crystalline zeolite containing catalyst, rapid deactivation of the zeolite can occur. This deactivation manifests itself in substantial measure as a loss of the crystalline zeolitic structure. This loss has been observed at vanadium levels of 1,000 ppm or less. The loss in the crystalline zeolitic structure becomes more rapid and severe with increasing levels of vanadium and at vanadium levels of about 5,000 ppm, particularly at levels approaching 10,000 ppm complete destruction of the zeolite structure may occur. Typically, the effects of vanadium deactivation at vanadium levels of less than 10,000 ppm can be reduced by increasing the addition rate of virgin catalyst, but it is financially costly to do so. As previously noted, vanadium poisons the cracking catalyst and reduces its activity. The literature in this field has reported that vanadium compounds present in feedstock become incorporated into the coke (which is deposited on the cracking catalyst), and is then oxidized to vanadium pentoxide in the regenerator as the coke is burned off (M. Xu et al. J. Catal. V. 207 (2), 237-246). At 700-830° C. in the presence of air and steam, Vanadium (“V”) will be in a surface mobile state in an acidic form. This Vanadium species reacts with cationic sodium, facilitating its release from the Y exchange site. The sodium metavanadate thus formed hydrolyzes in steam to form NaOH and metavanadic acid, which may again react with Na+ cations. V thus catalyzes the formation of the destructive NaOH.
Iron and nickel on the other hand are not mobile. The nickel containing hydrocarbons deposits on the catalyst and forms nickel oxide in the regenerator. In the riser section it may be reduced to metallic nickel, which, like metallic iron, catalyzes the dehydrogenation of hydrocarbons to form undesired hydrogen and coke. High hydrogen yields are undesirable because it can lead to limitations in the FCC downstream operations (the wet gas compressor is volume limited). High amounts of coke can otherwise lead to regenerator air blower constraints, which may result reduced feed throughput.
Because compounds containing vanadium and other metals cannot, in general, be readily removed from the cracking unit as volatile compounds, the usual approach has been to trap and/or passivate these compounds under conditions encountered during the cracking process. Trapping or passivation may involve incorporating additives into the cracking catalyst or adding separate additive particles along with the cracking catalyst. These additives combine with the metals and either act as “traps” or “sinks” for mobile Vanadium species, so that the active component of the cracking catalyst is protected, or act as passivators to immobilize Ni and Fe. The metal contaminants are then removed along with the catalyst withdrawn from the system during its normal operation and fresh metal trap is added with makeup catalyst, so as to enable a continuous withdrawal of the detrimental metal contaminants during operation. Depending upon the level of the harmful metals in the feedstock, the quantity of additive may be varied relative to the makeup catalyst in order to achieve the desired degree of metals trapping and/or passivation.
Modified FCC catalysts that incorporate various types of alumina is well known. For example, commonly assigned U.S. Pat. Nos. 6,716,338 and 6,673,235, which add a dispersible boehmite to the cracking catalysts. Upon calcination, the boehmite is converted to a transitional alumina phase, which has been found useful in passivation of nickel and vanadium contaminants in the hydrocarbon feedstock. Meanwhile, high surface area aluminas may also serve to trap vanadium and protect the zeolite. However, the high surface area aluminas do not passivate vanadium, so that the level of contaminant hydrogen and coke remains high.
Vanadium can also be trapped and effectively passivated by using alkaline earth metal containing traps (Ca, Mg, Ba) and/or Rare earth based traps, see the commonly assigned and co-pending application Ser. No. 12/572,777; U.S. Pat. Nos. 4,465,779; 4,549,958; 5,300,469; 7,361,264 However, these traps are sensitive to sulfur, and sulfur could block to active sites for vanadium trapping to make them less effective.
Usage of antimony and antimony compounds as passivators are also well known in the patent literature, see U.S. Pat. Nos. 3,711,422; 4,025,458; 4,031,002; 4,111,845; 4,148,714; 4,153,536; 4,166,806; 4,190,552; 4,198,317; 4,238,362 and 4,255,287. Reportedly, the antimony reacts with nickel to form a NiSb alloy, which is difficult to reduce under riser conditions, thus deactivating nickel for catalyzing the formation of hydrogen and coke. This process is commonly referred to as passivation.
In the commonly assigned U.S. Pat. No. 7,678,735, the addition of an ammoxidation catalyst to the FCC regenerator is described as reducing the emissions of NOx and NOx precursors during FCC catalyst regeneration. A particular useful ammoxidation catalyst is a mixed oxide of iron antimony and an additional metal, such as Mg, Mn, Mo, Ni, Sn, V or Cu. There is no mention in the patent of the specific utility of an ammoxidation catalyst in cracking of resids, and in particular, in the trapping and/or passivation of nickel and vanadium contaminants which can poison and/or deactivate the zeolite cracking catalyst.